1. Field of the Invention
The present invention relates to a rolling bearing. More particularly, the present invention relates to a rolling bearing for engine auxiliary machinery (alternator, magnetic clutch, compressor, intermediate pulley, water pump) which is used in an environment where the contamination with water makes it difficult to form a lubricant film.
2. Description of the Related Art
In recent years, with the reduction of the size and weight of automobiles, engine auxiliary machinery having a reduced size and weight as well as higher performance and output has been required. Accordingly, when the engine is actuated, the bearing, e.g., for alternator is simultaneously acted upon by high vibration and high load (about 4 to 20 G as calculated in terms of acceleration of gravity) accompanying high speed rotation through the belt. As a result, the outer bearing race as fixed race suffers early flaking that causes the reduction of-bearing life.
The early flaking is attributed to the rise in stress due to high vibration or the resulting difficulty in the formation of lubricant film that causes the decomposition of water content present in an amount of about 0.1% in the grease and hence makes it easy for surface contact to occur.
As a case where a rolling bearing is contaminated with water to show a reduced life there is disclosed in J. A. Cirura et al., xe2x80x9cWear, 24 (1973) 107-118, The Effect of Hydrogen on the Rolling Contact Fatigue Life of AISI 52100 and 440C Steel Ballsxe2x80x9d that a four-ball rolling test with a lubricant contaminated with water shows the reduction of life to one tenth of the initial value and a rolling fatigue test on a hydrogen-charged steel ball shows that stainless steel ball (SUS440C) has a longer life than ball made of bearing steel of the second kind.
However, the use of SUS440C as a bearing material for the purpose of prolonging the bearing life is disadvantageous in that SUS440C can hardly be worked as compared with the bearing steel of the second kind and forms on the rolling surface an eutectic carbide having a size of more than 10 xcexcm that adversely affects the rolling life and acoustic properties of the bearing, making it difficult to put SUS440C to practical use.
As a technique for prolonging the life of rolling bearings, e.g., for the foregoing engine auxiliary machinery, to be used with high vibration under high load there is disclosed a grease-filled bearing made of martensite stainless steel in at least a fixed race and 14% or 18% Cr-based high carbon stainless steel as in JP-A-3-173747 (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d).
Further, there is disclosed, among grease-filled bearing races, a rolling bearing made of steel containing from 1.5 to 6% of Cr in at least the fixed bearing race as in JP-A-5-26244.
As a countermeasure against early flaking there is disclosed in xe2x80x9cSAE Technical Paper: SAE950944 (held on February 27 to Mar. 2, 1995)xe2x80x9d, Articles 1 to 14, a technique resulting from the elucidation of the fatigue mechanism of bearing for alternator which comprises changing the filling grease from E grease to M grease having a high damping effect from which a lubricant film that can sufficiently absorb high vibration and high load is formed to prevent the rolling elements and the bearing races from coming in metallic contact with each other and hence early flaking.
In recent years, however, the development of small-sized high output alternators has been under way. This has caused the rise in the working temperature at which the bearings must be used to higher than 150xc2x0 C. Accordingly, even the improved M grease occasionally can hardly form a lubricant film. Further, since there occurs a great difference in environmental temperature, dew condensation can easily take place in the bearing, occasionally contaminating the interior of the bearing with water.
The bearing disclosed in the former of the foregoing citations (JP-A-3-173747) comprises 13Cr-0.7C steel which is a martensite stainless steel having a tempered martensite structure, 18Cr-1%C SUS440C or SUS420J2, which is 13Cr-0.3C steel, from which a passive film is formed to exhibit an enhanced resistance to hydrogen embrittlement. However, SUS420J2 is disadvantageous in that it exhibits HRC of 52, which is not high enough against rolling fatigue. SUS440A and SUS440C are disadvantageous in that they have a C content of more than 0.6% that causes the deposition of coarse eutectic carbide particles having a size of more than 10 xcexcm on the rolling surface. As a result, surface-starting flaking takes place. At the same time, the passive film can easily break, deteriorating the corrosion resistance of the bearing. Further, the use of these steel materials are disadvantageous in that it deteriorates the workability in the production of bearing.
Further, the bearing disclosed in the latter of the foregoing citations (JP-A-5-26244) comprises from 1.5 to 6% of Cr incorporated at least in the fixed race, thereby forming on the surface of the bearing races an oxidized Cr film that inactivates the rolling surface and hence making it possible to prevent the decomposition of grease resulting in the production of hydrogen that then contaminates the interior of the bearing. As disclosed in xe2x80x9cNihon Tripology Kaigi Yokoshu (Preprint of Japan Council of Tripology) (Tokyo, 1995-5)xe2x80x9d, pp. 551-554, however, a bearing for engine auxiliary machinery is liable to rotational slippage of rolling elements on the inlet side of the load zone of the fixed race. Thus, the oxidized Cr film (FeCrO4), which can be formed from a composition having a Cr content of from about 1.5 to 5%, can be easily cut when its thickness is from about 1 to 2 nm. Further, since the outer race, which is liable to frequent early flaking, is subject to direct application of load, the outer race cannot be prevented against early flaking.
Further, austenite steel such as SUS304 and SUS316, which exhibit a good acid resistance, lacks sufficient hardness, which is the most important factor for rolling bearing, and thus cannot be applied to bearing for engine auxiliary machinery, which is subject to high vibration and high pressure.
As described in WEAR199 (1996) 245-252 xe2x80x9cOccurrence of brittle flaking on bearing used for automotive electrical instruments and auxiliary devicexe2x80x9d, on the other hand, Tamada et al. reports that when subjected to linear contact rolling fatigue test on a test specimen which has been subjected to hydrogen charge test, a 13% Cr stainless steel test specimen undergoes structure whitening and breakage at the maximum shear stress generating position, suggesting that hydrogen embrittlement cannot be completely prevented.
Through studies of the mechanism of breakage of these 13%Cr stainless steels, the inventors found that the passive film does not prevent the penetration of hydrogen as has been heretofore insisted but the principle of adsorption of hydrogen atom by the passive film, i.e., mechanism that the amount of hydrogen adsorbed by the material itself increases but the penetration of hydrogen into the interior of the bearing, i.e., the maximum shear stress generating position can be prevented exerts the best effect of prolonging the bearing life.
It was also found that the material which can only form a passive film having a thickness as insufficient as from 1 to 2 nm and SUS440C, which forms an eutectic carbide having a size of 10 xcexcm on the surface of bearing, exert an adverse effect of easily allowing hydrogen atom thus adsorbed to penetrate into the interior of the bearing.
The present invention has been worked out on the basis of the foregoing knowledge. An object of the invention is to provide a rolling bearing which comprises a dense passive film on the race surface to a uniform thickness of from 5 to 100 nm to allow the surface layer to adsorb hydrogen atom, thereby preventing water from penetrating into the maximum shear stress generating position, comprises carbon and nitrogen incorporated in the material composition according to a specified relationship to enhance the abrasion resistance of the passive film, thereby preventing film breakage, and comprises carbon and chromium incorporated therein according to a specified relationship to reduce the rolling life or prevent the production of a large amount of coarse eutectic carbide particles having a size of 10 xcexcm or more that hinders the uniform formation of passive film, thereby fairly preventing early flaking even under severe working conditions where the bearing is subject to high temperature and vibration and contamination with water and hence prolonging the life thereof.
In order to accomplish the foregoing object of the invention, the rolling bearing according to the invention comprises a plurality of rolling elements disposed between a fixed race and a rotary race, wherein at least the fixed race is made of a steel comprising from 0.35 to 0.55% by weight of C, from 11.0 to 17.0% by weight of Cr, from 0.05 to less than 0.2% by weight of N and the rest of Fe and unavoidable components, with the proviso that the sum of the content of C and N is from 0.45% by weight to 0.65% by weight, and exhibits a surface hardness HRC of 57 or more after heat treatment and the diameter of eutectic carbide produced on the bearing races is 10 xcexcm.
Preferably, the rolling bearing comprising a plurality of rolling elements disposed between a fixed race and a rotary race, wherein at least the fixed race is made of a steel comprising from 0.35 to 0.55% by weight of C, from 11.0 to 17.0% by weight of Cr, from 0.05 to less than 0.2% by weight of N, from 0.2 to 1.0% by weight of Mn, from 0.2 to 1.5% by weight of Si and the rest of Fe and unavoidable components, with the proviso that the sum of the content of C and N is from 0.45% by weight or more to 0.65% by weight or less, which has been hardened, and then tempered at a temperature of from 400xc2x0 C. to 550xc2x0 C. to allow fine carbide particles having a size of from 50 to 300 nm to be secondarily deposited on the rolling surface so that the resulting surface hardness HRC is 57 or more, the diameter of eutectic carbide particles produced on the bearing race is 5 xcexcm or less and the amount of retained austenite is 6% or less.
More preferably, in the foregoing rolling bearing, the space in the bearing is filled with a urea-based grease made of a synthetic oil comprising 50% or more of ether oil as a base oil. In this arrangement, even under high vibration, the lubricity retention can be kept good, making it possible to inhibit the decomposition of water in the grease. Further, a high damping effect can be exerted. Accordingly, the passive film having a thickness of from 5 to 100 nm on the surface of bearing race can be fairly protected, making it possible to further prolong the rolling life.